ECG Basics
- JMD
- Dec 2, 2024
- 3 min read
Updated: Apr 19
When a muscle contracts, it causes electrical changes called "depolarization," which can be detected by electrodes on the skin. Even though the heart has four chambers, electrically, it can be simplified into two. This is because the two atria contract together, followed by the two ventricles contracting together.
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Each heartbeat begins with an electrical signal in a special area of the right atrium called the sinoatrial (SA) node. This signal (depolarization) spreads through the atrial muscle, causing the atria to contract. There is a slight delay as the signal passes through another area in the atrium called the atrioventricular (AV) node. After this, the signal travels quickly down the bundle of His, which runs through the wall (septum) between the ventricles and splits into right and left bundle branches. The left branch further divides into two.
Finally, the signal spreads more slowly through the ventricular muscle via Purkinje fibers, ensuring the ventricles contract efficiently.
The heart's electrical activation doesn’t always start in the SA node. The term "rhythm" refers to the part of the heart that controls the sequence of electrical activation. When the electrical signal starts in the SA node, the heart is in its normal rhythm, known as sinus rhythm.
THE PARTS OF AN ECG
The P wave on an ECG represents the contraction (depolarization) of the atria. A larger deflection, called the QRS complex, occurs when the ventricles contract (depolarize). The T wave represents the ventricles returning to their resting state (repolarization).
The letters P, Q, R, S, and T were chosen randomly in the early days of ECG development. Each letter corresponds to a wave, while the Q, R, and S waves together form the QRS complex. The period between the S wave and the start of the T wave is called the ST segment.
Sometimes, an extra wave, the U wave, appears after the T wave. It might represent the repolarization of papillary muscles. A U wave after a normal T wave is usually harmless, but if it follows a flattened T wave, it could indicate a problem.
Key points about the QRS complex:
A downward deflection at the start is a Q wave.
An upward deflection is always called an R wave, even if there’s no Q wave before it.
A downward deflection after an R wave is an S wave, regardless of whether a Q wave is present.
The PR interval represents the time for electrical excitation to travel from the atria to the ventricles. If the PR interval is very short, it may mean that the signal is bypassing the AV node or that there is abnormally fast conduction from the atria to the ventricles.
The QRS complex duration shows how long it takes for the ventricles to become excited (depolarized). Normally, the QRS duration is 120 ms (three small boxes on an ECG).
The QT interval changes with the heart rate and can become prolonged due to certain electrolyte imbalances or specific medications. If the QT interval exceeds 450 ms, it can increase the risk of a dangerous heart rhythm called ventricular tachycardia.
The six limb leads—I, II, III, aVR, aVL, and aVF—provide views of the heart's electrical activity from different angles in a vertical plane, encompassing perspectives from the sides and feet.
While many ECG machines automatically generate reports with measurements like heart rate and conduction intervals, these automated interpretations can sometimes over-diagnose abnormalities. Therefore, it's beneficial to develop confidence in your own ECG analysis.
A systematic approach to interpreting an ECG involves the following steps:
Heart Rate: Determine the heart rate.
Rhythm: Assess the regularity and origin of the heartbeat.
Conduction Intervals: Measure intervals such as PR, QRS, and QT to evaluate the electrical conduction system.
Cardiac Axis: Determine the heart's electrical axis to identify any deviations.
QRS Complexes: Analyze the morphology of the QRS complexes for signs of ventricular abnormalities.
ST Segments and T Waves: Examine these segments and waves for indications of ischemia or other cardiac conditions.
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